The use of liquid developers in electrophotographic printing machines is known. Liquid developers have many advantages, and often result in images of higher quality than images formed with dry toners. The toner particles can be made very small without resulting in problems often associated with small particle powder toners, such as machine dirt which can adversely affect reliability, potential health hazards, limited crushability, and restrictions against the use of coarsely textured papers. Development with liquid developers in full color imaging processes also has many advantages, such as a texturally attractive print because there is substantially no height build-up, whereas full color images developed with dry toners often exhibit height build-up of the image where color areas overlap. Further, full color prints made with liquid developers can be made to a uniformly glossy or a uniformly matte finish, whereas uniformity of finish is difficult to achieve with powder toners because of variations in the toner pile height, the need for thermal fusion, and the like.
Ideally, such liquid developers should be replenishable in the particular equipment in which they are used. In general, high solids concentration toners are used for replenishment because relatively low concentrations (e.g., in the range of 10 to 15% by weight solids) result in greater liquid build-up in the equipment, which then must be removed and disposed of as hazardous waste. Thus, it is desirable to initially use a toner containing less liquid, and to maintain the working source located within the equipment, thereby minimizing the undesirable accumulation of carrier liquid in the equipment. In addition, it is highly desirable and economically attractive to have the liquid vehicle containing the toner particles to be recovered economically and without cross contamination of colorants.
The application of liquid developer to the photoconductive surface clearly depletes the overall amount of liquid developer in the reservoir of an electrocopying or electroprinting machine of this type. In practice, the liquid reservoir is continuously replenished, as necessary, by addition of two liquids from two separate sources, the one providing carrier liquid and the other-a concentrated dispersion of toner particles in the carrier liquid. This is necessary in order to maintain in the carrier liquid in the reservoir a relatively constant concentration of toner particles, because the total amounts of carrier liquid and toner particles utilized per electrocopy vary as a function of the proportional area of the printed portions of the latent image on the photoconductive surface. An original having a large proportion of printed area will cause a greater depletion of toner particles in the liquid developer reservoir, as compared to an original with a small proportion of printed area. Thus, in accordance with the aforementioned practice, the rate of replenishment of carrier liquid is controlled by monitoring the overall amount or level of liquid developer in the reservoir, whereas the rate of replenishment of toner particles (in the form of a concentrated dispersion in carrier liquid) is controlled by monitoring the concentration of toner particles in the liquid developer in the reservoir. An optical float can combine both these functions, i.e. can be utilized to monitor both the overall amount of liquid developer in the reservoir and the toner particle concentration therein. The amount of toner particles utilized per electrocopy varies in proportion to the relative printed area of the image. Thus, a large number of so-called "white" copies (i.e. originals with small printed areas) will result in very small depletion of toner particles whereas the amount of carrier liquid depleted will be comparatively large.
It has been found that it is highly desirable to employ carrier liquid compositions and, in particular, to liquid developers comprised of a mixture of high and low vapor pressure fluids, and wherein there is enabled with such developers excellent fixing characteristics especially when the developed image is transferred from an intermediate substrate to the final substrate, such as paper, reference for example U.S. Pat. No. 5,276,492, the disclosure of which is totally incorporated herein by reference. U.S. application Ser. No. 08/461,829 entitled "LIQUID DEVELOPER COMPOSITIONS" the disclosure of which is totally incorporated herein by reference discloses developers and processes for achieving high fix wherein the developers contains a high vapor pressure fluid, such as an Isopar, like ISOPAR L.RTM., and a low vapor pressure fluid, such as Norpar 15, Superla NF5, and the like, and which low vapor pressure fluid is substantially odorless. The high vapor pressure fluid in embodiments is removed by heat once the developer is transferred to the intermediate substrate, and the low vapor pressure fluid remains with the developer when the developed image is transfixed, that is transferred, fixed and heated simultaneously, to a supporting substrate like paper.